1. Area of the Art
This patent application relates to voltage regulators and more particularly relates to voltage regulators for terminators such as in busses.
2. Description of the Prior Art
Computer systems typically use an electronic bus to communicate signals between various computing devices such as a processor, a memory, and input/output (I/O) devices. A computer bus commonly communicates address, data, and control signals between the computing devices connected thereto. The signals are typically driven on the bus by drivers incorporated into each of the computing devices connected thereto.
A common computer peripheral interface is the Small Computer Systems Interface (SCSI). The SCSI bus is becoming increasingly popular because it reduces the I/O bottle neck in existing computer systems. However, as the data rates over the SCSI bus increase, and as the distances between the computer peripherals connected to the bus also increase, transmission line effects associated with the SCSI bus degrade the integrity of the data transmitted over the bus. Additionally, impedance mismatches between bus terminators and cable lines cause signal reflection and power loss. Reflections can also result in unintended assertions or negations, causing false data or addresses to be provided to components coupled to the bus.
The SCSI-2 computer bus requires operation at 10 megabytes per second over a bus cable length of 6 meters. Termination has become a critical design factor due to the increases in data rates, the potential distances between SCSI stubs, differences in cable design and other factors. For example, due to impedance mismatches on the SCSI bus, the acknowledge and request control signals may be reflected causing the acknowledge and request lines to be double clocked. One source of signal reflection is due to mismatches between cables having slightly different impedances. Another source of reflection is due to the "stubs" (i.e., the length of cable that is coupled to the primary SCSI bus) and the position of the stubs on the SCSI bus.
The SCSI physical interface contains specifications for maximum cable length, maximum number of devices, minimum and maximum cable impedance, maximum load capacitance, minimum driver rise/fall time, maximum termination current, minimum open circuit termination voltage, and various other requirements to maintain high bus speeds. The voltage waveform on the bus has a square wave characteristic and the voltage levels are TTL. The open circuit voltage of the terminator must be a minimum of 2.5 volts and the maximum output current is 24 milliamps for any voltage above 0.2 volts.
Meeting the SCSI specifications should minimize or eliminate problems from signal reflection due to impedance mismatching. However, this ideal is not met in practice but is approximated with varying extents by various manufacturers of terminators. Such approximations have included both passive terminators and active Boulay and current mode terminators. Unfortunately, while the specification can be readily satisfied on an individual component basis, in mass production there is great difficulty in maintaining tolerances sufficient to meet the specification. Consequently, most efforts in improving the performance of the SCSI bus have been directed at reducing the deleterious effects of the reflection problems associated with the SCSI bus. Various termination techniques have been attempted.
For example, passive terminators have been used for terminating single-ended SCSI-1 devices. FIG. 1 shows a typical passive terminator which provided reliable operation even when the SCSI bus was fully configured and run at maximum cable lengths. As shown in FIG. 1, the passive terminator 100 terminates a bus signal line 102 into a resistive load consisting of a 220 ohm resistor 104 connected to the terminator power line 106 and a 330 ohm resistor 108 connected to ground. The effective resistance of the passive terminator is equal to 132 ohms. However, the passive terminator 100 provided a resistive path between the terminator power line 106 and ground even when the signal line 102 is not active (i.e., at high impedance), resulting in continuous power dissipation even when all of the bus signal lines 102 are negated. For a terminator power voltage of 5.0 volts, the passive terminator 100 dissipates 50 mW (10.00 milliamps.times.5.0 volts) for every inactive bus signal line 102. Another disadvantage of the passive terminator 100 shown in FIG. 1 is that the Thevenin voltage is not regulated and thus varies with variations in the terminator power 106. For example, a terminator power variation between 4.25 volts and 5.25 volts causes the output voltage to vary from 2.55 volts to 3.15 volts. Consequently, a correspondingly large variation in the current supplied to an asserted bus signal line (e.g., signal line 102) through the 220 ohm resistor 104 is produced. Precision tolerance resistors (+/-1% or less) are required in order to limit the output current provided on bus signal line 102, thereby making the manufacture of the passive terminator 100 costly.
Active bus terminators, such as the Boulay terminator shown in FIG. 2, have also been developed. Active termination of the computer bus provides a potential reduction of reflection problems caused by impedance mismatches on the bus. In general, the prior art active terminators attempt to reduce the reflection by compensating for voltage drops and maintaining a constant stable voltage to the terminating equipment resistors. The Boulay terminator 200 shown in FIG. 2 uses an active voltage regulation technique to improve noise immunity and reduce average power dissipation. The linear voltage regulator 202 produces a voltage source of 2.85 volts on line 204. As shown in FIG. 2, the 2.85 volts is provided in series with a plurality of terminating resistors 206 which are connected to a plurality of computer bus signal lines 208. Typically, the plurality of terminating resistors 206 comprise 110 ohm resistors, having a 1% or better tolerance. The scheme shown in FIG. 2 is suited to terminate bus lines having a relatively low characteristic impedance, which is fairly common. Because the computer bus signal lines 208 are terminated by an active voltage regulation scheme, noise immunity is improved and a substantial reduction and average power dissipation results. The Boulay terminator reduces average power dissipation because a deasserted or high impedance line conducts no current through its respective terminating resistor 206. Thus, the only power dissipated by the Boulay terminator 200 for the negated line is the power dissipated by the linear voltage regulator 202. Typically, the linear voltage regulator 202 dissipates between 5.0 and 10.0 milliamps of current.
Furthermore, because the Thevenin voltage is regulated, the output current is substantially immune to variations in termination power. Disadvantageously, in order to provide the maximum current on the computer bus signal lines 208, the terminating resistors 206 typically must be high precision resistors. When the resistors are included on an integrated circuit device together with the regulator 202, laser trimming is required to produce resistors 206 having these low tolerance values. Consequently, the prior art Boulay terminators had high manufacturing costs.
FIG. 3 shows one attempt at providing a controllable bus terminator with voltage regulation at a plurality of termination impedances 398 connected through switchable elements such as bipolar transistors to the output of the voltage regulator. The voltage regulator 306 includes a differential amplifier 320, a transistor 340, a switchable current source 350, and a pass transistor 342. The emitter of the pass transistor 342 is coupled to the collectors of the switching transistors 300 as shown. The differential amplifier 320 cooperates with the transistor 340 and the switchable current source 350 in order to control the conduction of the pass transistor 342 and thereby hold the regulator's output voltage 327 constant. This regulated voltage is coupled to switching transistors 300 that couple the regulated voltage to the termination impedances 398. These transistors can either be conducting during operation or be open to disable the terminator.
To address the problem of transmission errors, double clocking of acknowledge and request lines, and effects of signal reflection, the embodiment of FIG. 3 employs a source only regulator and clamp circuitry. The source only regulator provides a current source when the cable lines 372 sink current away from the terminator. The clamp circuitry sinks current when the cable lines 372 source current into the terminator causing the voltage at the regulated node to drift upward.
Each bus line 372 includes a voltage clamp which has a clamping transistor 378 and a current source 374 connected through a diode 376 to ground 312. The base of clamping transistor 378 is connected between the current source 374 and diode 376. The emitter of the transistor 378 is connected to the bus line 372. If signal reflection sources current into the bus line 372, potentially causing the signal on the bus line 372 to negate, the clamping transistor 378 conducts to clamp the voltage on the bus line 372 at zero volts.
It is common to use push/pull drivers to negate a line or totem pole drivers to assert or negate so that the bus lines can be driven high or low rapidly. FIG. 4 shows an embodiment of a bus terminator employing totem pole drivers. The power supply VSS is coupled through a voltage regulator to a plurality of active terminators. The totem pole drivers 424a, 424b, . . . and 424n are coupled to the respective cable lines. Each of the totem pull drivers 424a, 424b, . . . and 424n have a structure with a first bipolar transistor coupled to the cable line at an emitter to provide a current source and a second bipolar transistor coupled to the cable line at a collector to provide a current sink. The cable lines are asserted when the second transistor sinks current from the cable line and negated when the first transistor sources current into the cable line.
When a bus terminator uses voltage regulation that sources current into the regulation node with push/pull or totem pole drivers, the line can be driven above the output voltage of the regulator with the line voltage drifting higher approaching the termination power supply voltage. This can cause the voltage regulator to lose regulation and make it more difficult to assert the line, often causing data transmission errors. To address this problem, prior art terminators have used voltage clamp circuitry, such the voltage clamps 522, to sink current when the voltage reaches a given voltage (somewhere above the regulator output voltage).
However, there are many problems using voltage clamp circuitry to combat the effects of signal reflection. First, clamping circuitry requires additional components. For example, the clamping circuitry of the embodiment of the FIG. 3 terminator employs a bipolar transistor 378 for each line and a current source 374. This requires a larger die area and consumes more power.
Second, clamping may result in a voltage deadband between the regulator output and the clamp circuit voltage. The clamping voltage must be above the desired reference voltage or the clamp will always be on, providing for higher current draw. If noise or reflections cause the cable voltage to drift above the desired voltage but below the clamping voltage, the drivers may not be able to negate the line, creating a deadband.
Consequently, there remains a need for bus terminators that do not require costly, high precision resistors. There is also a need to avoid the undesirable effects of clamping circuitry to overcome the problems of signal reflection due to impedance mismatching.